This invention relates to the field of spinal surgery. More specifically, this invention relates to apparatuses for creating cavities in vertebral bodies and in intervertebral discs located between the vertebral bodies. This invention also relates to methods for creating such cavities. Once the cavities are created with the apparatuses and according to the methods of the present invention, an intervertebral prosthetic device, designed to replace a damaged intervertebral disc, can be implanted in the cavities. Moreover, the implanted device may be used in vertebral body fusion or in reconstruction of mobile discs through spinal arthroplasty (i.e., disc replacement).
The human spine is a flexible structure comprised of twenty-five vertebrae. Intervertebral discs separate and cushion adjacent vertebrae. The intervertebral discs act as shock absorbers and allow bending between the vertebrae.
An intervertebral disc comprises two major components: the nucleus pulposus and the annulus fibrosis. The nucleus pulposus is centrally located in the disc and occupies 25-40% of the disc""s total cross-sectional area. The nucleus pulposus usually contains 70-90% water by weight and mechanically may function like an incompressible hydrostatic material. The annulus fibrosis surrounds the nucleus pulposus and resists torsional and bending forces applied to the disc. Thus, the annulus fibrosis serves as the disc""s main stabilizing structure. A healthy disc relies on the unique relationship of the nucleus and annulus to one another. The top and bottom surfaces of intervertebral discs abut vertebral body endplates.
Individuals with damaged or degenerated discs often experience significant pain. The pain results, in part, from instability in the intervertebral joint due to a loss of hydrostatic pressure in the nucleus pulposus, which leads to a loss of disc height and altered loading of the annulus fibrosis.
A conventional treatment for degenerative disc disease is spinal fusion. In one such surgical procedure, a surgeon removes the damaged natural disc and then fuses the two adjacent vertebral bodies into one piece. The surgeon fuses the vertebral bodies by grafting bone between the adjacent vertebrae and sometimes uses metal rods, cages, or screws to hold the graft in place until the graft heals. Other fusion procedures do not require surgical removal of the disc.
Although spinal fusion may alleviate pain associated with degenerative disk disease, it also results in loss of motion at the fused vertebral joint. Lack of motion at the fused site puts abnormal loads on the adjacent discs above and below the fusion. This additional pressure may cause the adjacent discs to degenerate and produce pain, thereby recreating the problem which originally existed. To remedy the problems associated with spinal fusion, various prosthetic devices were developed to replace the damaged disc with a suitable biomechanical equivalent.
Existing prosthetic devices have met with limited success in reproducing the biomechanics of a natural disc. For example, U.S. Pat. No. 4,759,769 to Hedman et al. discloses a synthetic disc having upper and lower plates hinged together. Although the hinged disc allows forward bending between adjacent vertebrae, the hinged disc does not allow axial compression or lateral flexion. Nor does it allow axial rotation of the vertebral column at the site of the implant. Therefore, the Hedman et al. device lacks many of the biomechanics of a natural disc.
Likewise, the prosthetic disc device disclosed in U.S. Pat. No. 4,309,777 to Patil does not replicate natural motion between adjacent discs. The Patil device includes two cups, one overlapping the other and spaced from the other by springs. The cups move only in a single axial dimension. Thus, the Patil device does not enable natural flexion of the spine in any direction. In addition, the highly constrained motion of the Patil device can lead to high device/tissue interface stresses and implant loosening.
Many synthetic devices connect to the vertebral bodies by conventional mechanical attachments, such as pegs or screws, which are known to loosen under cyclic loading conditions. Other synthetic devices use plastic or elastomeric components which, over a lifetime, produce debris from wear and possible unknown side effects.
In response to these and other known problems associated with synthetic prosthetic disc devices, U.S. Pat. No. 5,827,328 to Buttermann, which is incorporated herein by reference in its entirety, discloses an intervertebral synthetic prosthetic device designed to replace the biomechanical functionality of a failing intervertebral disc. One embodiment of the Buttermann device includes a first fixation member for implantation in a first vertebral body, a second fixation member for implantation in a second vertebral body adjacent the first vertebral body, and a compressible member that is positioned between the first and second fixation members. The Buttermann device overcomes the aforementioned problems with synthetic devices.
There is a need for improved apparatuses and methods by which cavities can be created in vertebral bodies and in an intervertebral disc. Once the cavities are created, an intervertebral prosthetic device designed to replace a damaged intervertebral disc, such as the one described in U.S. Pat. No. 5,827,328, can be implanted in the cavities.
In one aspect of the present invention, a cutting guide is provided for use in removing bone from a vertebral body. The cutting guide includes a sidewall that defines an internal cavity. In addition, the sidewall has (i) a first edge to face toward and to contact a vertebral body in at least three points and (ii) a second, opposite edge to face away from the vertebral body. The first edge includes at least two concave portions and at least one convex portion oriented generally perpendicular to the at least two concave portions.
The sidewall of the cutting guide may be comprised of four walls arranged to form a rectangular cross-section. At least one of the four walls may include a hole extending from the first edge to the second edge to receive a fastener therethrough. Further, the first edge of the sidewall may be concave along a first of the four walls and along an opposite second of the four walls, and it may be convex along a third of the four walls and along an opposite fourth of the four walls. The concave first edge along the first wall may be a mirror image of the concave first edge along the second wall. Similarly, the convex first edge along the third wall may be a mirror image of the convex first edge along the fourth wall. Moreover, although the concave and convex edges may each comprise one smooth surface, they also may be formed by a plurality of adjacent surfaces.
In another aspect of the invention, a chisel guide is provided for use in cutting bone of a vertebral body. The chisel guide includes a first block member to be positioned adjacent the vertebral body and a second block member connected to the first block member. The second block member has a channel, formed on one side thereof, which terminates at the first block member.
In one embodiment of the chisel guide, the first block member and the second block member may be formed as one integral piece. Further, the second block member may extend beyond the perimeter of the first block member in at least one dimension to form a shoulder with the first block member. For example, the second block member may have a width greater than the width of the first block member such that the second block member forms a pair of opposed shoulders with respect to the first member.
Another aspect of the invention relates to a cutting guide and chisel guide combination for use in removing bone from a vertebral body. The combination includes a cutting guide having a sidewall defining an internal cavity. The sidewall, in turn, has a first edge to face toward and to contact a vertebral body in at least three points and a second, opposite edge to face away from the vertebral body. The combination also includes a chisel guide. The chisel guide has a first block member and a second block member connected to the first block member. The first block member is adapted to be inserted into the internal cavity of the cutting guide to position the first block member adjacent the vertebral body such that a passage remains between a first side of the first block member and an inner surface of the sidewall of the cutting guide.
In the aforementioned cutting guide and chisel guide combination, the first and second block members may be formed as an integral piece. Moreover, the second block member may be solid. In addition, the second block member may extend beyond the perimeter of the first block member, in at least one dimension, to form a shoulder with the first block member. For example, the second block member may have a width which is greater than the width of the first block member so that the second block member forms a pair of opposed shoulders with respect to the first block member. Finally, the second block member may include a channel which is formed on one side thereof and which terminates at the first block member.
In yet another aspect of the invention, an apparatus for use in removing bone from a vertebral body is provided. This apparatus includes a shaft and a reamer. The shaft has a first end and a second end, and the second end of the shaft is connectable to a power source. The reamer is connected to the first end of the shaft. The reamer includes at least one cutting member and a collection space to collect bone fragments cut by the cutting member. A slot, through which the bone fragments pass into the collection space, is adjacent the at least one cutting member.
The reamer may be detachably connected to the first end of the shaft. In addition, the reamer may have a circular cross-section. Various power sources, such as a drill, may be used to rotate the second end of the shaft. In one embodiment, the cutting member is positioned on a bone engaging surface of the reamer. The bone engaging surface of the reamer may be flat, except for the cutting implement and slots associated therewith.
In still a further aspect of the invention, an apparatus for creating a cavity in a vertebral body endplate and in an intervertebral disc is provided. The apparatus includes a handle, a first arm, and a second arm movable toward the first arm upon actuation of the handle. The apparatus also includes a first cutting implement that is mounted to the first arm and that has a generally circular sidewall that terminates in a first cutting edge. The first cutting edge faces away from the first arm. In one embodiment of the cavity creating apparatus, the first cutting edge also may face away from the second arm. In another embodiment, the first cutting edge may face toward the second arm.
In addition, the apparatus may also include a second cutting implement mounted to the second arm and having a generally circular sidewall that terminates in a second cutting edge, the second cutting edge facing away from the second arm and facing toward the first cutting edge so that, upon actuation of the handle, the first and second cutting edges move toward each other. The cutting edges of the cutting implements may be serrated. The cutting implements also may be rotatably mounted to their respective arms. Moreover, in an embodiment having two cutting implements, the cutting implements may be mounted to rotate about the same axis of rotation.
In yet a further aspect of the invention, a tensioner apparatus for use in determining a proper elongation distance in a prosthesis implanted in a vertebral body is provided. The tensioner apparatus includes a first arm and a second arm, each having a handle portion and a separator portion. A pivot joins the first arm to the second arm and separates the separator portions from the handle portions. In addition, at least one tension measuring element is positioned on the first arm; the tension measuring element may be a strain gage.
The tensioner apparatus also may include at least one strain gage positioned on the second arm. Moreover, the strain gages positioned on the first and second arms may be part of a Wheatstone bridge and may be positioned on the separator portions of the first and second arms, respectively.
The invention also contemplates a method of creating a cavity in a vertebral body. The cavity creating method includes removably attaching a cutting guide to an outer surface of a vertebral body. The cutting guide has a cavity therein. The method also includes puncturing through the outer surface and cortical bone of the vertebral body along a perimeter of the cavity in the cutting guide, removing the punctured cortical bone of the vertebral body to expose bone in the interior of the vertebral body, and removing the bone in the interior of the vertebral body.
The method of creating a cavity in a vertebral body may also include inserting a chisel guide in the cavity in the cutting guide. In this method, the puncturing step may include using the chisel guide to guide a chisel, having a chisel blade, along a perimeter of the cavity in the cutting guide. Alternatively, the puncturing step may be accomplished by using a motorized sagittal saw. Regardless of whether a chisel and chisel guide or a sagittal saw is used to puncture through the outer surface and cortical bone, the step of removing the bone in the interior of the vertebral body may be accomplished using a reamer.
The invention further contemplates a method of creating an intervertebral disc cavity. The method includes providing an apparatus including a first arm having a first cutting implement attached thereto, a second arm, and a handle. The method also includes positioning the first arm in a cavity in a first vertebral body, compressing the handle of compressor so that the first and second arms move in a direction towards each other, cutting through the nucleus pulposus of the intervertebral disc with the first cutting implement, and removing the nucleus pulposus of the intervertebral disc to create the intervertebral disk cavity. This method also may include positioning the second arm of the compressor in a cavity in a second vertebral body, wherein the second arm has a second cutting implement thereon; and cutting through the nucleus pulposus of the intervertebral disc with the second cutting implement.
The method for creating an intervertebral disc cavity may also include, prior to the step of positioning a first arm of a compressor in a cavity in a first vertebral body, attaching a cutting guide to a surface of the first vertebral body, the cutting guide defining a cavity; cutting through the surface and cortical bone of the first vertebral body along an inside perimeter of the cavity in the cutting guide; removing the cut cortical bone of the first vertebral body to expose bone in an interior of the first vertebral body; removing the bone in the interior of the first vertebral body to create the first vertebral body cavity; and removing the cutting guide from the first vertebral body. Further, the method also may include attaching the cutting guide to a surface of the second vertebral body; cutting through the surface and cortical bone of the second vertebral body along the inside perimeter of the cavity in the cutting guide; removing the cut cortical bone of the second vertebral body to expose the bone in the interior of the second vertebral body; and removing the bone in the interior of the second vertebral body to create the second vertebral body cavity.
In addition, the invention contemplates a method of applying a predetermined load to an implanted device. The implanted device has a fixation member implanted within a vertebral body and a compressible member implanted within an intervertebral disc. The method includes providing a tensioner including a first arm and a second arm each having a handle portion and a separator portion. A pivot pin joins the first arm to the second arm and separates the separator portions from the handle portions. At least one strain gage is positioned on the first arm. The method also includes inserting the first and the second arms into the fixation member, and moving the handle portions toward each other to thereby move the separator portions away from each other until one of the separator portions contacts an upper member of the fixation member and the other of the separator portions contacts a lower member of the fixation member. The method further includes elongating the fixation member with the tensioner, and monitoring a voltage measured by the at least one strain gage, the voltage being representative of the load applied by the tensioner and thus the reactive load experienced by the compressible member of the implanted device.